Archive for the ‘Repairs’ Category

A couple of weekends ago I took the two XB-2s that I had (at the time) over to Ron’s shop to have a little more room to spread out and test things. Like real Hammond tonewheel organs, the XB-2 has “drawbars” that represent different harmonics (or subharmonics) of the fundamental frequency of each key being pressed; you draw out the bars to mix different amounts of the different harmonics to get the timbre you want. This is additive synthesis at its most visceral.

On the XB-2, the drawbar positions (either live or recalled from memory) are displayed on an LCD below the manual (keyboard). In live mode, the bar graphs move in and out in synchrony with the physical drawbars.

On one of the two XB-2s, the LCD bar graphs didn’t match the drawbars — a couple of drawbars appeared to work properly, but some didn’t work at all and others moved multiple bar graphs on the display. Since the drawbar decoding is a relatively independent section of the organ, it seemed like an easy repair to tackle first.

The drawbar’s wiring harness plugs into the main board on J121, at the left of this section from the service manual. Each drawbar is a detented slide potentiometer, so variable voltages are arriving on J121. The section enclosed in the dotted line and marked not used truly isn’t populated on the circuit board, so I omit it from discussion.

The nine analog drawbar voltages are delivered to IC23 and IC24 (TC4051 analog multiplexers). The multiplexers receive their enable and select signals from the output of IC29 (74HC174 hex D flip-flop) which is latching signals previously delivered from the system data bus. (In other words, the 74HC174 is the drawbar select register; its own address is decoded elsewhere in the schematic.)

The chosen (enabled) TC4051 analog mux selects which input to pass to its output on pin 3, which is then op-amp buffered and delivered to the input pin of IC25 (BA9101 analog-digital converter). When selected (more system address bus decoding), the ADC writes the digital value of the drawbar’s position onto the system data bus.

Side note: For the drawbars only having nine detents (0-8), IC25 sure delivers a lot of bits of ADC resolution to the data bus.

I put a scope on IC23 (analog mux)’s output pin and I was able to view on the screen the time-division multiplexing of the drawbar positions (analog voltages) onto the single line going to the ADC. It mostly matched what I saw on the LCD, although there were some quirks with a few of the drawbar time divisions appearing narrower than others. Ignoring the odd widths and recording which drawbar occupied which time division:

Drawbar Time-Division Multiplexing Behavior

Drawbar

16

8

4

2

1

5 1/3

2 2/3

1 1/3

1 3/5

IC23 Pin

13

14

15

12

1

5

2

4

(IC24)

IC23 Input

Good MB

0(000)

1(001)

2(010)

3(011)

4(100)

5(101)

6(110)

7(111)

(IC24)

Bad MB

0(000)

1(001)

7(111)

7(111)

0(000)

1(001)

7(111)

7(111)

(IC24)

On the working XB-2 motherboard, the drawbars were selected and sampled in numerical order. On the broken motherboard, as you can see, any time the analog mux’s select bit A1 was enabled, the mux behaved as though bits A2 and A0 were enabled as well. Further, select bit A2 didn’t work on command as it should when drawbars 4-7 should have been chosen.

4051 Address Pins

Name

C

B

A

Function

A2

A1

A0

Pin

9

10

11

It could be a bad 4051 mux; but as we had already replaced a leaky electrolytic capacitor in the neighborhood, it seemed worth another look at the circuit board first. The 4051′s select lines are on pins 9-11, and what’s this?!

I became suspicious of a damaged via on a trace that turned out to connect to pin 9 (A2). A continuity test showed that the via — even its top side — was no longer connected to IC23; the trace up to the via had been eaten away by the leaking capacitor. The via — even its top side — did have continuity to its next stop on the PCB, so the via itself was intact.

Ron heated the solder that had wicked into the via during reflow, inserted a piece of wire-wrap wire, and soldered the other end directly to IC23 pin 9. The drawbars now work perfectly. I suspect the floating select input on the CMOS mux was picking up enough signal from the PCB trace inductively coupled to its neighbor to trigger.

My hypothesis is that the previous owner put the keyboard away because of larger (ROM / CPU / Muse) problems; the capacitor leaked and damaged the drawbar multiplexer trace while it was sitting idle; and the owner never even knew about the drawbar problem. At any rate, it was easily fixed and the troubleshooting was a rewarding mental exercise.

I have two secondhand Hammond XB-2 Hammond clones (c’mon, even if it had made by Hammond proper before being bought by Suzuki, a solid-state, digital audio synthesis keyboard is still a clone) which both appear to have bit rot in their firmware EPROMs, apparently a common problem with these keyboards.

I’d be extremely grateful for pointers to where I could get new EPROMs or download images to burn myself, or to another XB-2 owner willing to read out their EPROMs to assist. Heck, I’ll burn you another copy so you have a fresher set when yours bit-rot.

My friend Jeremy has a 1995 Mustang that had the factory premium sound system in it when he bought the car used. The CD player was broken and he had the head unit replaced within a couple of weeks of owning the car. He later added a subwoofer.

I’ve always thought the stereo lacked clarity in the bass, and the head unit and EQ have had some quirks. Recently Jeremy pulled the head unit and found all sorts of interesting techniques used by the aftermarket installer that will be the subject of a later monologue … but one of the things we discovered is that the amplifier for the door woofers wasn’t working at all. Swapping it with the amp for the rear deck woofers caused them to go silent and the (shot) door woofers to work again (after resoldering their cut cables).

Both my Crumar T1 and T2 “portable” organs (the lower two cases in the stack) came to me without swell (volume) pedals. Each has a rotary potentiometer on its control panel for master volume, but I really want to be able to change the volume dynamically while playing. I’ve been using a Dunlop volume pedal (built into a rocker case identical to the CryBaby wah) on the organ’s output; but (at least when used with the organ) all of the pedal’s action is in about the lower quarter of its physical range, so it’s very finicky to use.

I recently bought this original T1 swell pedal on eBay, listed as untested / project. That usually means tested / didn’t work / can get more money if I don’t admit that I already know it doesn’t work; but I figured I could fix whatever was wrong with it. And I have.

Bad horsie 2 that was plugged into the wrong power supply and messed up, and needs some minor electronic work.

I was intrigued by the challenge (I’m such a sucker for broken things, dang it) and bought it. When the seller and I exchanged the pedal for my cash, he remarked that he read on a forum that it probably just needed a resistor changed, and that if I were handy with a soldering gun I could probably do it myself.

Uh huh. Resistor.

Let’s dig in.

The circuit board has a hole in the top for a foam battery “cage” attached to the enclosure, something clever that I haven’t seen before. And it had no obviously damaged components.

A while back, I bought a secondhand Liebert GXT2-2000RT120 uninterruptible power supply (UPS) on eBay. The GXT2 is a series of online UPSes, meaning that the output power always comes from the inverters off the battery bank; it doesn’t switch from utility power to battery power like an offline UPS. Besides eliminating any possible switchover glitches, online UPSes always deliver conditioned power at a constant voltage. The 2000RT120 is a 2000VA unit with 120V output — large enough to power all my servers for a good little while.

The batteries were due for replacement and the seller removed them to save on shipping costs. I got a UPS with a set of wires and no instructions on how to connect them.

Also one of the wires was compromised … but since it appears to be a ground wire, I figured no big deal if it shorts out against the cage. KIDDING!

Yesterday I figured out the wiring, installed batteries, and got the UPS set up in my server rack.

Recently a couple of pieces of audio processing equipment I’ve bought used have had bad left channels. After recognizing the pattern, I finally thought to swap out the patch cord I had left plugged into the “test” channel on my keyboard mixer, and voila! Left channels fixed.

I’ve always been curious about the construction quality of commercial patch cords — just how good are the connections buried under those lovely molded jackets and strain reliefs?

Naturally, the faulty end was the last one I disassembled. (Logic joke!)

I have a whole box of what I believe to be moderate- to high-quality over-the-ear headphones with worn-out, shredded cords.

Anyone know a source for replacement headphone cables, preferably straight (not coiled) with 1/4″ plugs, and absolutely with supple cable and alreadying Y-ing out to both ears? 6′ cable would be okay and 10′ would be fantastic.

I recently bought a DBX 266XL audio compressor/limiter on eBay. The seller described it thus:

Has light scratches, small amount of rack rash, in perfect working condition- no issues whatsoever. Has been used in my guitar rig for the past several years with no problems.

It arrived oddly but adequately packed and … as you can see, not in perfect condition. I would go so far as to say it had issues. I suspect had I tried to use it, I would have had problems.

Well … I could complain to the seller, who would tell me it was damaged in shipping, and then I could try to deal with the USPS who I don’t think broke it, and I could spend a lot of time and frustration and maybe get some money back and probably end up with no compressor. Or I could just fix it myself and have a little fun in the process.

As mentioned previously, I recently bought a Crumar T2 organ manufactured in 1978 and started ascertaining its condition. Here’s what I’ve been able to fix so far and what I’ve been able to determine about the parts I haven’t yet fixed.

Crackly Volume Knobs and Stuck Master Tuning Potentiometer

Several of the volume knobs were pretty crackly.

Most Crumar keyboards are wonderful to service because of how easy it is to get inside. After removing a few screws, the top panel lifts back on its rear hinge, without even having to take the knobs off all the controls.